Petroleum-based resins such as polyethylene terephthalate (PET), nylon, polyolefin or plasticized polyvinyl chloride (PVC) are widely used today for a variety of applications, for example, a packaging material. However, such petroleum-based resins are not biodegradable, thereby causing environmental pollutions, e.g., emission of a large amount of greenhouse gases during waste disposal processes. Recently, due to gradual depletion of petroleum resources, the use of biomass-based resins, typically polylactic acid resins, is extensively considered as an alternative.
Particularly, in recent years, there has been growing interest in food packaging films made of biodegradable, heat sealable films. For this purpose, amorphous polylactic acid resins based on optical isomers in combination of D-lactide and L-lactide monomers have been developed.
However, such amorphous polylactic acid resins to serve as a base resin for heat sealable films may suffer from poor extrudability during the formation of films caused by blocking that may occur at temperatures above their glass transition temperatures. Polylactic acid resins show low adhesiveness, when heat sealed, because of their unique low cohesive attraction. They also have issues associated with storage instability due to their limited storage conditions. Accordingly, polylactic acid resins have limited applicability due to their poor adhesiveness, processability, and storage stability, as compared with conventional linear low density polyethylene (LLDPE) resins commonly used as a heat sealable resin.
Further, polylactic acid resins do not have satisfactory mechanical properties as compared with petroleum-based resins. They also involve a problem of low flexibility when formed into films. In order to redress such problems, a number of methods have been suggested: for example, adding to a polylactic acid resin a low molecular weight softener or plasticizer, introducing a plasticizer prepared by addition polymerization of a polyether- or aliphatic polyester-based polyol, etc. Most of the packaging films prepared from polylactic acid resins by such methods, however, still have limited flexibility. Moreover, said plasticizers may bleed out over a period of time and give rise to a drawback that packaging films prepared therefrom suffer from high haze and low transparency. Thus, in recent years, it has been suggested to prepare a block copolymer by introducing a polyurethane polyol repeating unit to a polylactic acid resin (see Korean Laid-Open Patent Publication No. 2013-0135758) in order to overcome these problems.
However, there is still a demand for improving such conventional polylactic acid resins with respect to desirable characteristics for film processing such as glass transition temperature, melting temperature, crystallization properties, and the like, as well as desirable mechanical properties for heat sealable films such as thermal adhesiveness, flexibility, and mechanical strength.